Provenance and sedimentary processes controlling the formation of lower Cambrian quartz arenite along the southwestern margin of Baltica

Abstract

Lower Cambrian shallow marine quartz arenite records a transgressive regime related to a global basal Cambrian eustatic sea-level rise. Six sections from southern Norway, southern Sweden, and Denmark are investigated to explore the genesis and sourcing of these mineralogically mature deposits and the early Cambrian tectono-sedimentary history of Baltica. U-Pb ages of detrital zircon grains are dominantly 0.9–1.8 Ga, in accordance with transport from the Transscandinavian Igneous Belt (TIB) and domains related to the Sveconorwegian and Gothian orogenies. These zircon grains have a hydrodynamic relation with the quartz grains, suggesting a common provenance for the zircon grains and the main clastic material. Similar provenance for Norway and southern Sweden favors a setting with the present day Sveconorwegian Orogenic Belt originally extending further southeast into the present Skagerrak and Kattegat area and northwest in westernmost Norway. Furthermore, marked provenance differences with other earliest Cambrian deposits on Baltica indicate a catchment divide between the Sveconorwegian Orogenic Belt and the TIB-Gothian domains. First sediment-cycle origin for the studied sandstone is proposed based on: 1) a general low age diversity and 2) a lack of late Palaeoproterozoic and Archaean zircon U-Pb ages, which are typical for Mesoproterozoic quartzite. The high sandstone maturity may instead be the result of prolonged exposure to weathering and reworking processes made possible by the low gradient of the Sub-Cambrian Peneplain.

Introduction

Thick quartz-arenitic successions are commonly assumed to be formed through recycling processes in high-energy marine and aeolian realms (Pettijohn et al., 1987, Johnsson et al., 1991, Cox and Lowe, 1995, Prothero and Schwab, 1996, Dott, 2003). In these environments with repeated and long-lasting reworking of sand grains, mechanical and chemical breakdown of feldspar and other silicate minerals results in a relative increase in quartz and other mechanically and chemically stable minerals (Blatt and Christie, 1963, Suttner et al., 1981).

Recycling of clastic particles derived from break-down of sedimentary rocks is estimated to account for 80% or more of the sedimentary system (Cox and Lowe, 1995, and references therein). Nevertheless, world-wide, several Precambrian and Cambro-Ordovician quartz arenite occurrences have been interpreted as first-cycle deposits. Some of these deposits were produced from intense weathering and erosion in tropical humid climate (Soegaard and Eriksson, 1989, Dott, 2003, Avigad et al., 2005) and subsequent transportation from a low-relief source area with low sedimentation rates (Pettijohn et al., 1972, Suttner et al., 1981; Basu, 1985, Chandler, 1988), in addition to aeolian processes and marine reworking. Quartz arenite has also been interpreted to originate from the dissolution of chemically unstable clastic minerals during burial diagenesis (Chandler, 1988, Went, 2013).

Lower Cambrian quartz arenite occurs on many continents, such as Baltica, Laurentia, Amazonia and Siberia (Goodwin and Anderson, 1974, Lindsey and Gaylord, 1992, McKie, 1993, Wahab, 1998, Sears and Price, 2003). Quartz arenite deposits are also part of Cambro-Ordovician occurrences on Gondwana (Avigad et al., 2005, Bassis et al., 2016). These continents formed as separate plates during the Neoproterozoic-Cambrian break-up of the supercontinent Rodinia (Li et al., 2008, Torsvik and Cocks, 2016). Active sea-floor spreading gave rise to early Cambrian eustatic sea-level changes with sea-level high stand and transgression (Haq et al., 1988), resulting in a lower Cambrian quartz arenite drape on denudated continents. These quartz-arenitic formations display many similarities in stratigraphy and petrography, although deposited on different continents (e.g. Swett et al., 1971, Lindsey and Gaylord, 1992, Avigad et al., 2005).

The Precambrian basement of Baltica includes Fennoscandia in the north and the East European Platform in the south. Baltica was surrounded by passive margins with the exception of the accretionary, latest Neoproterozoic Timanian orogen in the north (Pease et al., 2008, Kuznetsov et al., 2014). It was separated from Siberia, Gondwana and Laurentia by the Ægir, Tornquist and Iapetus oceans, respectively (Hartz and Torsvik, 2002, Cocks and Torsvik, 2005, Torsvik and Cocks, 2005). Furthermore, a rift succession developed along the western margin of the continent during Neoproterozoic time, during which the Sub-Cambrian Peneplain also formed as a low-relief denudation surface on Baltica (Lidmar-Bergström, 1993, Gabrielsen et al., 2015). Transgressive, shallow-marine quartz-arenitic sandstone is found directly on the peneplain (Nystuen, 1982, Kumpulainen and Nystuen, 1985, Moczydłowska and Vidal, 1986, Nystuen, 1987, Nielsen and Schovsbo, 2011).

For Baltica, the cause of high mineralogical maturity has been little discussed in previous studies: is the maturity a function of multicyclic reworking? Or first-cycle sand formed during processes of weathering and denudation of basement rocks? Baltica included felsic magmatic and metamorphic rocks as well as Palaeoproterozoic-Neoproterozoic quartz arenite (Singh, 1969, Lidmar-Bergström, 1993, de Haas et al., 1999, Bingen et al., 2001, Laajoki et al., 2002, Andersen et al., 2004, Bingen et al., 2008a). Therefore, the provenance is of significance when discussing the formation of the lower Cambrian quartz arenite on Baltica.

Provenance studies of Neoproterozoic-Cambrian sandstone from the Scandinavian part of Baltica are scarce. The studies are mainly restricted to U-Pb age determinations of detrital zircon grains from a Cryogenian-Ediacaran succession along the northwestern Baltica margin, that was included in lower and middle Caledonian nappe units during Silurian time (e.g. Bingen et al., 2005, Be'eri-Shlevin et al., 2011, Bingen et al., 2011, Lamminen et al., 2015). Detrital zircon ages are also available for Ediacaran-Cambrian sandstone or quartzite of the autochthonous to parautochtonous part of central and northern Norway (Andresen et al., 2014, Zhang et al., 2015), as well as for Cambro-Ordovician shale from southwestern Norway (Slama and Pedersen, 2015, Slama, 2016) and quartzite from Denmark (Olivarius et al., 2015).

The method of using U-Pb ages in provenance studies have been criticised for being too low in precision to distinguish source rocks of various continents, and for being insufficiently specific in relating protosources to identifiable rocks in a crystalline basement (Andersen, 2014, Kristoffersen et al., 2014). It has also been emphasised that detrital zircons can only be used to deduce the opposite direction of sediment transport, i.e. from ‘sink’ to ‘source’ (Andersen et al., 2017). In accordance with this statement, we apply U-Pb ages of detrital zircons to identify protosource provinces of the clastic detritus, in combination with analysis of sediment distribution as function of structural framework and sedimentary processes in the present study. The objective of this study is to obtain better knowledge and understanding of the genesis of the high maturity in lower Cambrian quartz arenite on the southwestern margin of Baltica. Emphasis is given to potential source rock areas and the effect of weathering and transportation processes to sediment dispersal. Furthermore, the aim is to improve the tectono-sedimentary history of Baltica, by identifying differences in provenance and clastic sediment routing for Norway, Sweden and Denmark.